(1) Field of the Invention
The present invention relates to spin coating apparatus for use in applying a photoresist or other coating solution in film form to surfaces of substrates such as glass substrates for liquid crystal displays, semiconductor wafers, or mask substrates for manufacturing semiconductor devices.
(2) Description of the Related Art
The spin coating apparatus applies a coating solution to a spinning substrate, and the solution is spread over the substrate by the centrifugal force. In the case of using the apparatus for a square substrate, the formed film tends to be thicker in the corners than in other parts of the substrate.
This is because the corners of a square substrate in a spin cut through ambient air or gas, and are exposed to the air more than other parts of the substrate. As a result, evaporation of the solvent of the coating solution is promoted in the corners to increase viscosity of the solution and diminish the spreading effect of the centrifugal force. In the case of a round substrate, the larger the substrate is, the greater is the difference in peripheral speed between a radially outward position and a radially inward position of the substrate. Thus, the tendency to increase the film thickness in peripheral regions is not negligible.
A spin coating apparatus devised to solve the problem encountered with square or large substrates is disclosed in Japanese Patent Publication Kokai (Unexamined) No. 1990-219213. This apparatus includes at least a chuck for holding a substrate and a spinning device for spinning the chuck. The substrate is spun in a horizontal plane as held by the chuck, whereby a coating solution once applied to the substrate is spread over an upper surface of the substrate. This known apparatus further includes an upper spinning plate having a larger diameter than the substrate. The spinning plate is disposed parallel to and slightly spaced from the upper surface of the substrate supported on the chuck. The substrate and spinning plate are spun together.
In the above prior construction, a layer of air over the upper surface of the substrate spins with the substrate and spinning plate as sandwiched therebetween. With the air little disturbed in peripheral regions of the substrate, the coating solution is applied in a uniform film thickness.
However, the conventional apparatus noted above has the following new disadvantages:
The upper spinning plate must be disposed as close to the substrate as possible in order to cooperate with the substrate to spin the layer of air over the substrate. Consequently, mist of the coating solution scattering upward from the substrate in the spin coating treatment adheres to and hardens on the lower surface of the upper spinning plate. In the course of repeated spin coating treatments, the lower surface of the spinning plate becomes rugged with the coating solution adhering thereto. It has been found that the rugged surface causes those disturbances of the layer of air over the substrate which will be detrimental to uniform film formation. Further, in a subsequent spin coating treatment carried out before hardening of the coating solution adhering to the lower surface of the upper spinning plate, this coating solution could scatter and contaminate the surface of the substrate.
It is therefore necessary in an actual operation of the treatment to detach and clean the upper spinning plate at appropriate intervals of time before the above inconveniences are encountered. Not only are the detachment, cleaning and reassembly irksome, but the discontinuation of the treatment results in a reduction in the treating efficiency as a whole.